1. Field of the Invention
The present invention relates to thermosetting resin compositions and, more particularly, to thermosetting resin compositions yielding flexible cured products exhibiting both a small coefficient of thermal expansion and a small degree of mold shrinkage.
2. Description of the Prior Art
Thermosetting resin compositions have excellent electrical properties, including dielectric properties, volume resistivity and insulation breakdown strength, and excellent mechanical properties such as flexural strength, compressive strength and impact strength. These resins are therefore useful insulating materials for electric or electronic parts that are fabricated by a variety of methods such as transfer molding, injection molding, potting, casting, powder coating, immersion coating and dipping.
Thermosetting resins are generally rigid resins. The coefficient of thermal expansion and shrinkage following molding exhibited by these resins are generally different from that of the electric or electronic part they protect. Electric and electronic part elements have relatively small coefficients of thermal expansion or low shrinkages while these values for the resins are relatively large. These large differences in expansion and shrinkage between the two materials generates excessive internal stress in the element or other structural material of the electric or electronic part after sealing, aftercure or subsequent thermal hysteresis. The stress may cause the part to malfunction or break. In addition, these differences in coefficients of thermal expansion or shrinkages may cause the thermosetting resin itself to crack or may cause voids between the electric/electronic part and the resin. The coated part is susceptible to deterioration when water or other material invades this void.
Although not directed specifically at improving the coefficient of thermal expansion or the shrinkage of molded thermosetting resins, there have been efforts to improve the properties of these materials. For example, Japanese Patent No. 52-36534[77-36534] describes an improvement in the lubricity of the surface of resin moldings by including a polyorganosilsesquioxane powder in the moldable composition. Japanese Kokai [Laid Open] Patent Publication No. 52-14643[77-14643] teaches increasing the wear resistance of synthetic resins with respect to metals by using as the filler a finely divided cured material composed principally of a polyorganosiloxane and an inorganic filler. Japanese Kokai [Laid Open] Patent Publication No. 48-25739 discloses adding cured silicone rubber in powder form to an epoxy resin. None of these methods provides an adequate improvement in the coefficient of thermal expansion, the shrinkage after molding or the flexural modulus of the cured resin.
Japanese Kokai Patent Publication No. 58-219218[83-219218] describes a method conceived by the present inventors to solve the above-described shortcomings whereby a finely divided cured elastomeric material containing at least 10 wt. % of a straight-chain siloxane fraction is blended into the thermosetting resin. The problem with this method is that it is not necessarily easy to finely grind the cured elastomeric material. In Japanese Kokai Patent Publication No. 59-96122[84-96122], the present inventors propose overcoming the disadvantages of prior art methods by using as the additive spherical particles of a material that cures into an elastomeric material. The particles are produced by spraying a curable composition in the form of spherical particles. This method is quite effective, however, the cost of the cured particles is high due to the requirement for a plant to produce them.
An objective of the present invention is to provide a thermosetting resin composition with good fluidity during molding, and which is easily released from a mold without soiling of the metal molds or exudation of a liquid onto the surface of the cured material. The cured materials exhibit good flexibility, a small coefficient of thermal expansion and a low shrinkage during molding.